We didn't bother with save the dates, we're just telling people as we see/speak to them, so I can't comment on how we did it.
Regarding other weddings that I've received 'save the dates' for, I'm not sure you need to put any names on at all!
I've usually just had a card with a 'X&Y are getting married on dd/mm/yy, save the date!'
It will save you time (from writing out names) and money (if you decided to get them printed with names) to avoid names altogether.
If you do decide to go now the names route, I would go with:
'NAME & NAME' if they are in a relationship,
'NAME, NAME & family' if they have kids that are going to be invited,
and just 'NAME' if they are single or in a new relationship, regardless of whether they will be getting a plus one - you may want to decide that closer to the time, and it's easier to give a surprise plus one where it's needed than to revoke one where it isn't!
Hope this helps! x